Safety switches or safety sensors serve for detecting the shut position of shutting parts, for example of doors or hinged covers, in relation to closeable parts, for example boundary walls, housing walls and the like provided with corresponding access orifices, and, for safety reasons, cooperate with a special actuator, so that their tripping position cannot be manipulated by means of simple tools or the like. Such safety switches comprise, in general, a switch unit and a trip, in each case one of these components being arranged on the closeable part and the other on the shutable part. In this context, safety switches with mechanical or magnetic, that is to say non-contact coupling between the switch unit and trip are known.
As regards mechanical coupling, exact alignment, along with low tolerances, are necessary, with the result that adjustment becomes more difficult. Since mechanical devices do not operate without contact, it is difficult to keep them free of germs, this being important, for example, for areas of use in the food sector.
Contactless safety switches based on interconnected Reed contacts are known. However, since Reed contacts weld together under relatively high currents or capacitive loads, the reliability of such safety switches is often inadequate, and, moreover, they require special evaluation units.
German Patent 4 112 064 discloses an alarm indicator arrangement with a switch unit which has a transmission circuit with an oscillator and with a transmission coil and a receiver circuit connected to a discriminator circuit and having at least one receiver coil. Here, continuous power transmission, even when signal transmission is taking place, is ensured. However, an arrangement of this kind does not always afford the required safety standard.
It is known from German Patent 3 029 543, in the field of signalling and monitoring technology, to use a primary unit in the form of a switch unit which possesses a transmission circuit with an oscillator and with a transmission coil and the receiver circuit connected to an evaluation unit and having at least one receiver coil and which cooperates with a secondary unit, a trip. The secondary unit comprises a receiver coil and a trip coil which is controlled by the transmission coil of the primary unit and which triggers an indicator signal in the shut position. In this case, a first frequency is transmitted from the primary unit to the secondary unit and a second frequency is transmitted in the reverse direction. However, an arrangement of this kind does not always afford the required safety standard, either.
German Laid-Open Patent Application 2 033 682 discloses a connecting device for electrical appliances, by means of which device a battery-powered appliance is brought into mutual bearing contact with a mains-powered appliance for signal or power transmission. Here, in each case a permanent magnet in one appliance closes a sealed-armature contact in the other appliance. A special coil with a U-shaped core is provided in each appliance for signal transmission and for power transmission in each case.
German Laid-Open Patent Application 4 113 665 discloses an external-field-safe contact arrangement of magnetically actuated switching elements for monitoring the shut state of windows and doors, a permanent magnet being mounted on a window wing or a door and at least two magnetic-field actuated switching elements being mounted on the window frame or door frame. The abovementioned problem of the welding together of contacts of the magnetically actuated switching elements arises here.